Since the first report of a semiconductor p-n junction solar cell from Bell Laboratories more than a half a century ago, silicon (amorphous, polycrystalline or monocrystalline) has dominated the photovoltaic market with nearly 99% market share. More than 90% of photovoltaic panels (sold and installed) are based on crystalline (mono and polycrystalline) silicon. For aesthetic and for budgeting reasons, crystalline based solar cells are not the most suitable photovoltaic technology. For this reason, the various thin film-based photovoltaic panels are expected to take over crystalline silicon as the dominant technology in the future. For example, thin film technology uses two orders of magnitude less photovoltaic material, and the process of fabrication is simpler. For crystalline silicon, the cost of photovoltaic materials and their processing accounts for up to 50% of the total cost of the device. Since processing crystalline silicon requires the use of high temperature (up to 2000° C. or more) and high vacuum, costs are expected to keep rising.
State of the art mono-crystalline solar cells provide power efficiency of about 24%. However, even after several decades of intense R&D the cost of electricity production from solar energy is still about 10 times higher than fossil fuel-based power generation. Silicon-based photovoltaic technology has greatly benefited from the development of mass production of high quality silicon wafers for the microelectronic industry. However, because of its indirect energy band gap, silicon is not an efficient light absorption material in comparison with direct band gap semiconductors with similar characteristics. Thus, relatively thick film (several hundred micrometers) of expensive high purity silicon is required to absorb about 90% of impinging solar radiation. In the case of thin film technologies, photovoltaic materials require films that are only 0.1 to 1 micrometer thick.
It is generally believed that the ideal solar cell material would have the following characteristics: (i) direct energy band-gap of about 1.4 eV; (ii) readily available raw materials; (iii) amenable to an easy and reproducible deposition technique suitable for large area production; (iv) high light absorption coefficient and good photovoltaic conversion efficiency; and, (vi) long-term stability.
Among alternative active materials to silicon, organics have received attention. Since the first report about 20 years ago of a homojunction (single layer) organic-based cell with an efficiency of 1%, several single layer thin film photovoltaic devices based on small molecules have been reported. Processing and tunability of organic-based photovoltaic materials have potential advantages over silicon in the development of affordable and efficient photovoltaic cells. Polymers may be processed and fabricated cost effectively. Furthermore, they potentially provide flexibility in chemical tailoring to obtain desired properties. Another advantage of organic materials is their high light absorption coefficient. In spite of these advantages, solar energy conversion efficiencies reported so far on organic photovoltaic cells are much lower than the 24% efficiency obtained with mono-crystalline silicon based solar cells.
Heterojunction active layers formed from a blended donor (D) acceptor (A) nanocomposite material where the length scale of the blend is similar to the exciton length have received some interest recently. Heterojunctions provide large interfacial area within a bulk material in which any point is within nanometers of the D/A interface. This may permit a near ideal charge separation and charge transfer. However, blending has not lead to a significant enhancement in power conversion efficiency. One of the main bottlenecks is the lack of efficient charge recuperation leading to significant charge recombination.
In spite of all the efforts geared toward the development of processable semiconductive polymers, carrier mobility is still about 3 orders of magnitude lower than inorganic semiconductors. This limits the efficiency of charge transfer to the electrode. The low intrinsic carrier mobility of plastic materials is considered to be the main obstacle in achieving high efficiency in organic solar cells.
Because of their high surface area, inorganic nanomaterials are also receiving attention. Hybrid nanomaterials may combine the high charge mobility of inorganic materials with the flexibility of the organic materials.
Use of nanostructured active material with high surface-to-bulk ratio, combined with possibility of optoelectronic properties tunability, have been proposed to enhance photovoltaic efficiency. The larger active surface area in hybrid nanocomposites allows maximizing the harvesting of incident solar energy. One possible avenue is to use nanostructured hybrid materials composed of solution based inorganic nanoparticles interfaced with conductive media. An overall conversion efficiency of nearly 8% with a 10 μm film composed of a few nm TiO2 particles coated with a dye has been reported. Although dye sensitized solar cell technology has attracted large interest from both academia and industry, it is still hindered by some stability issues. Replacing the solution medium with a polymeric matrix has led to some promising devices. A solid-state dye-sensitized solar cell (DSSC) consisting of three active layers (organic dye as light absorber, nanocrystalline metal oxide as electron transporter and an organic hole transporting film) has been reported to have a power conversion efficiency of 2.5%.
A blend of CdSe nanorods and a conjugated polymer has been reported to provide a power conversion efficiency of 1.7%. Good photovoltaic response (external quantum efficiency of 20%) on a hybrid film of hole conductor CuInS2 nanoparticles imbedded in an electron conductor matrix has also been reported. However in such photovoltaic materials, the only component that permits tunability is the inorganic semiconductor component. The energy gap of the inorganic semiconductor material is tunable by varying nanocrystal particle size without changing the remaining properties (such tunability has not been demonstrated even in inorganic semiconductor based solar cells).
All inorganic photovoltaic nanocomposites-based solar cells have been reported providing advantages over hybrid nanocomposites. However, laboratory power conversion efficiencies are still low, and fabrication processes are not scalable.
There remains a need for semiconductor materials having good power efficiency based on thin film technology.